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Lecture 6


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[Front]


what is an isotope
[Back]


same element, same amount of protons, but different amount of neutrons and tus different masses

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Lecture 6 - Marcador

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Lecture 6 - Detalles

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What is an isotope
Same element, same amount of protons, but different amount of neutrons and tus different masses
Stable isotopes do not
Decay over time
What is the oxygen (d18O) reference standard called?
VSMOW = Vienna Standard Mean Ocean Water
Why do we need isotope reference standards?
Absolute isotope quantity is difficult to measure accurately, however, relative differences can be measured accurately. Common reference point/baseline to ensure accuracy, reliability and reproducibility of isotopes data from different laboratories and studies. Calibration, interlaboratory comparison, and quality control of procedures.
The two most relevant types of isotopic fractionation are:
Kinetic (results when rates or reactions or physical processes differ, e.g., biosynthetic pathways) and equilibrium (occurs because the thermodynamic properies of isotopically substituted species differ e.g., fractionation betwee inorganic carbon species in water)
During kinetic isotopic fractionation, eutotrophic organisms fractionate ... 13C durig inorganic carbon uptake (CO2) in different degrees
Against (epsilon value works in the negative direction, larger number is larger fractionation)
What is rubisco ?
Earth's most abundant enzyme, used by autotrophic organisms to convert CO2 into organic compounds via the Calvin-Benson pathway
What factors influence fractionation in autotrophs
Growth rate (fractionation is reduced at faster growth rates, more product), concentration of CO2 (more CO2 = pickier, more fractionation) & cell geometry (smaller, pickier, more fractoination)
Fractionation for rubisco:
(straight chain) lipids (alkyl) most depleted, then isoprenoids and sugars are most positive (unused substrate)
Reversed TCA - occurs in some bacteria and some archaea. Fractionation:
Reverses tge way they produce compound. So sugars most depleted, then isoprenoid lipids then straight chain lipids/alkyl
Reductive acetyl CoA pathway, not cyclic pathway - used by some bacteria (eg sulphate reducers ) and archaea (methanogens). Fractionation:
Very large fractionation compared to CO2, straigh chain lipids/alkyl & isoprenoids is super depleted vs biomass. Methanogens super depleted in d13C and dD(deuterium/hydrogen) as methane is aswell
Anammox fractionation by bacteria
Relatively large fractionation, isoprenoid and alkyl lipids (straight chain) strongly depleted in 13C versus biomass (NH3 & NO2- into N2)
3-Hydroxy propionate pathway, used e.g. by phototrophic green non-sulphur bacteria. Fractionation
Uses bicarbonate instead of CO2, it is more positive by about 7-8 permille than CO2 – so bacteria using this will already be more positive from the start -> biomass lil. depleted vs CO2, alkyl enriched vs CO2, isoprenoids more depleted compared to alkyl and biomass
C4 plants live in ... regions and are generally ... compared to C3 plants
Dry (predominantly grasses on savannahs) & enriched -> C4 plants have concentrating mechanism (limited exchange with atmosphere) -> fractionation does not depend on CO2
We can measure our biomarker lipid and then reconstruct C3/C4 plant n-alkanes can be used to reconstruct higher vegetation and differentiate between C3/C4, but
Different contributions of different plant types with different 13C contents may bias the overall n-alkane isotope patterns -> no quantification possible, but at least reconstruct some changes in vegetation
Methanotrophs
Very depleted in d13C and dD
Nitrogen isotopes are used for ... studies while carbon isotopes are used for ... studies
Food web studies (position)(because nitrogen increases by 3.5 permillle per trophic level and 13C much less) & food source
N2 -fixation
Hardly any isotopic fractionation, slightly negative N isotopes
Denitrification (mainly perfeormed by anaerobes)
Generates 15N-depleted N2 resulting in 15N-enrichment of remaining nitrate
What is this and what is it a biomarker of?
Porphyrins - nitrogen isotopes
What are the two stable isotopes of hydrogen making of the dD?
Protium (1H) and deuterium (2H, D). -->deuterium has one proton, one neutron and one electron
DD is measured against?
SMOW (Standard Mean Ocean Water) and SLAP (Standard Light Antarctic Precipitation)
DD has a much larer range than d13C because?
Because of the larger mass difference
Clouds are ... in deuterium
Depleted (due to the evaporatio-precipitation balance and fractoination)
Rain is ... compared to the cloud
Enriched (heavier will rain out first, leaving the cloud more depleted)
Largest step in hydrogen isotopic fractionation in organisms is by formation of .... and estimated to be ca. 171 per mill
NADPH - but internally there is more fractionation taking place, but it is hard for us to know which enxymes/pathways are responsible
Hydrogen isotopic fractionation in plants: Lipid classes have different hydrogen isotopic compositions (but differently from 13C), name the pattern
N-alkanoic acids least depleted, then sterol, then lastly phytols (compared to water!) -> compared to bulk: phytols least depleted and then n-alkanoic&sterols
Where it is synthesised in the cell determines ....
The starting pool of hydrogen isotopes
DD of n-alkanes (from plants) mostly reflects ...
Precipitation/evaporation balance because this affects the dD of the source water
Influences on dD of terrestrial n-alkanes
Source water dD(precip/evap), fractionation due to biosynthesis of lipids and minorly by C3/C4 metabolism
Lower dD = ?
More rain out(wet climate)/fresh water
Hydrogen fractionation is impacted by light
More fractionation at light optimum
The hydrogen isotopic composition of lipids from aquatic organisms is influenced by the following major factors:
- Metabolism of organisms - Biosynthesis lipids - dD of the source water - Salinity – in marine environment - Light – we like to ignore this - Only minor effect of growth rate/nutrients
The bomb spike (spike in 14C) is usefull
Because the spike is represented in all material of that age and acts as a great tracer
Suess effect ?
Depletion in d14C due to input of large amounts of fossil fuel derived CO2 (contains not 14C) (occurred before bomb spike due to burning of fossil fuels, 1950s)
Compare radiocarbon of biomarker lipids from surface sediments with surface waters...
If they have same composition, they are from surface waters, if lipids are more depleted they originate from bottom waters and made them there
Compound specific radiocarbon can be used to elucidate the age of a biomarker and particularly its origine e.g. ..... ?
Local vs laterally transported? depth in watercolumn (Was it in contact with atmosphere? Surface ocean? Deep ocean?)? Influx from land?